This invention relates to a process for the recovery of noble metals, and particularly the recovery of noble metals which have volatilised or become detached in a very finely divided state from the surface of noble metal catalyst gauzes used in high pressure exothermic gas reactions.
U.S. Pat. No. 2,920,953 in the name of A. Rudorfer discloses that noble metals, particularly metals of the platinum group, which volatilise from the surface of noble metal catalysts in exothermic gas reactions, may be recovered if a granular layer of acid-soluble oxides of alkaline earth metals or of heavy metals, preferably of oxides which have been formed in situ from the corresponding carbonates, for example marble, is placed in the reaction furnace on the outlet side of the noble metal catalyst surfaces. The noble metal thus precipitated on this granular material, in the case of platinum, for example as the platinate, then may be recovered in a simple manner by dissolving the granular intercepting material in acid. In the above mentioned patent it is also recommended, inter alia, that the granular mass should be covered on the gas inlet side with a foil of material stable at red heat, provided with holes or slits, and it is also explained that a decrease in the gas resistance may be achieved by a so-called "arrangement of the resistance", that is to say by enlarging the surface for the passage of gas by giving the intercepting layer, for example, a wavy or tubular shape. A bed thickness of 160 to 170 mm is recommended for the charge of intercepting material.
While the aforesaid known process has proved successful in practice for the recovery of noble metals in processes conducted under normal pressure, on the other hand, in processes which operate under increased pressure, for example pressures of 1 to 10 atmospheres gauge, difficulties have arisen, above all because the granular mass was so caked and consolidated within a short period of time, that the gas resistance of the intercepting layer reached an intolerable level. The recovery process therefore could not be used in practice in processes operating under raised pressure. On the other hand, however, recovery of noble metal is of particular interest precisely in high pressure processes for ammonia combustion in view of the substantially higher losses of platinum (1.5 g of Pt/metric ton of nitrogen at 5 atmospheres gauge as against 0.2 g of Pt/tN at 0.2 atmosphere gauge).